A particularly troublesome characteristic of single-frequency semiconductor diode lasers is that when the current or temperature of the laser is modulated, both the intensity of the output power and the frequency, or equivalently, the wavelength of the laser output radiation change. These changes are referred to as intensity modulation (IM) and frequency modulation (FM). Wavelength modulation is called WM. In the telecommunications industry, where the encoding technique is generally on-off keying, engineers would like laser to have all IM and no FM, for FM creates cross talk among the closely spaced wavelength-division-multiplexing channels of the system. Conversely, in trace gas sensing, one would like to frequency-modulate the laser without any variation in the intensity. IM creates interfering signals, and introduces noise into the measurement system. Therefore, it would be desirable to have a single-frequency laser that exhibits FM with either current or temperature tuning, while eliminating the IM.